No he can't. That would be a violation of MCO P1020.34F
its really a use descression. As a Marine of two years i would not think it was appropriate to wear the dress blue uniform to a high school graduation as a graduate. I think if he has someone that he should of gotten guidance from a recruiter or someone.
If you want to look at the regulation specifically then yes in that that way, you would be unable to wear the appropriate headgear in uniforn, covered in doors....
Bottomline he should of gotten guidance on it before he said anything to the school.